1. Field of the Invention
The present invention relates to an electromagnetic wave absorber which is formed of Mn—Zn ferrite and which has an excellent absorption performance in a high frequency band.
2. Description of the Related Art
An electromagnetic wave absorber to absorb electromagnetic waves may leverage either ohmic loss of a resistive element, dielectric loss of a derivative, or magnetic loss of a magnetic substance. In case of an electromagnetic wave absorber leveraging magnetic loss, its absorption characteristics can be evaluated by reflection coefficient calculated using a formula (1) below:
                              Reflection          ⁢                                          ⁢          Coefficient                =                ⁢                  20          ⁢          log          ⁢                                                                                  Z                  in                                -                                  Z                  o                                                                              Z                  in                                +                                  Z                  o                                                                                                                  Z          in                =                ⁢                              Z            o                    ⁢                                                    μ                r                                            ɛ                r                                              ⁢                      tanh            ⁡                          (                              j                ⁢                                                                  ⁢                                                      2                    ⁢                    π                                    c                                ⁢                f                ⁢                                                                  ⁢                d                ⁢                                                                            μ                      r                                        ⁢                                          ɛ                      r                                                                                  )                                          where μ is permeability, ∈ is permittivity, c is light velocity, f is frequency of an electromagnetic wave, d is thickness of an electromagnetic wave absorber, and Z is characteristic impedance. Generally speaking, an electromagnetic wave absorber, which has a reflection coefficient of 20 dB or more at a given frequency band, is evaluated to be sufficiently absorbent in the frequency band.
An electromagnetic wave absorber leveraging magnetic loss may be formed of a ferrite material, such as an Mn—Zn ferrite or an Ni—Zn ferrite. An Mn—Zn ferrite is used mainly for an electromagnetic wave absorber intended to absorb electromagnetic waves in a relatively low frequency band ranging from 30 to 500 MHz, while an Ni—Zn ferrite is used for an electromagnetic wave absorber intended to absorb electromagnetic waves in a relatively high frequency band ranging from 500 MHz upward. Since an Ni—Zn ferrite is expensive, it is desirable to use a less expensive Mn—Zn ferrite also for a high frequency band application.
In an electromagnetic wave absorber formed of an Mn—Zn ferrite, an eddy current flows increasingly in accordance with an increase in frequency therefore causing an increase of loss. Accordingly, the resistivity of an Mn—Zn ferrite must be increased in order to duly absorb electromagnetic waves in a high frequency band. When an Mn—Zn ferrite containing more than a stoichiometric composition of 50.0 mol % Fe2O3 is sintered, Fe3+ is reduced to produce Fe2+ and an electron transfer occurs easily between Fe3+ and Fe2+, whereby the resistivity decreases to fall below 1.0 Ωm. Consequently, an Mn—Zn ferrite can be used as an electromagnetic wave absorber in a frequency band only up to a few hundred kHz, from which upward an Mn—Zn has its permeability lowered significantly and loses soft magnetic characteristics thus failing to function as an electromagnetic wave absorber.
In order to increase resistivity, an Mn—Zn may contain CaO, SiO2 or the like as additive for increasing electrical resistance of its crystal grain boundary and at the same time may be sintered at a low temperature of about 1200 degrees C. for reducing its crystal grain size from about 20 μm to about 5 μm thereby increasing the ratio of crystal grain boundary. In such an Mn—Zn ferrite, however, since the crystal grain boundary itself has a low electrical resistance, it is difficult to gain a resistivity of more than 1.0 Ωm. Also, if 0.20 mass % or more CaO is added, an abnormal grain growth occurs at sintering and its characteristics are deteriorated significantly.
An Mn—Zn ferrite with an increased resistivity is disclosed in, for example, Japanese Patent Application Laid-Open No. H09-180925, which contains base components of 20.0 to 30.0 mol % MnO, 18.0 to 25.0 mol % ZnO, and the remainder consisting of Fe2O3, and which has a DC resistivity of 0.3 Ωm or more, and a permittivity ε of 100000 or less at 1 kHz. The Mn—Zn ferrite is made to achieve an increased electrical resistance by adding CaO, SiO2, SnO2 and/or TiO2 thereto, but can thereby achieve a resistivity of only up to 2.0 Ωm, which is still not good enough to absorb electromagnetic waves in a high frequency band.
Another Mn—Zn ferrite is disclosed in, for example, Japanese Patent Application Laid-Open No. H07-230909, which contains base components of 45.0 to 48.6 mol % Fe2O3, an appropriate mol % (to constitute a sum of 50.0 mol % together with Fe2O3) Mn2O3, 28.0 to 50.0 mol % MnO, and the remainder consisting of ZnO, and further contains 0.01 to 0.50 mass % SiO2 and CaO as additive, and in which 1.0 mol % or less (0 excluded) Fe2+ is present. The Mn—Zn ferrite is for use as a magnetic core material of a deflection yoke and is made to achieve an increased resistivity by limiting Fe2O3 content to less than 50.0 mol %. The Mn—Zn ferrite is intended for application to a frequency band of 64 to 100 kHz and not suitable for use in a high frequency band exceeding 1 MHz.
And, still another Mn—Zn ferrite is disclosed in, for example, Japanese Patent No. 3108803, which contains base components of 44.0 to 50.0 mol % (50.0 excluded) Fe2O3, 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % TiO2 and/or SnO2, and the remainder consisting of MnO, and which has an electrical resistance of 150 Ωm or more. The Mn—Zn ferrite is made to achieve an increased resistivity by limiting Fe2O3 content to less than 50.0 mol %.
In order to well function in a high frequency band, an electromagnetic wave absorber must exhibit appropriate characteristics with regard to permeability and permittivity as well as resistivity. An electromagnetic wave absorber formed of a conventional Mn—Zn ferrite can function only in a limited frequency band, and therefore a conventional Mn—Zn ferrite sintered alone cannot make an electromagnetic wave absorber adapted to function in an extensive frequency range including a high frequency band.